Fire crews and residents in the Blue Mountains have had an exhausting night as they battled to protect homes ahead of worsening fire conditions.

An aggressive back-burning operation is being carried out to try and prevent three major blazes from merging into a massive single front in the coming days.

But the efforts have been hampered, with firefighters along the Bells Line of Road forced to protect homes coming under ember attacks.

An emergency warning remains in place for the massive State Mine fire near Lithgow, while watch-and-act alerts continue for blazes near Mount Victoria, Springwood and the Hall Road fire in the Southern Highlands.

A state of emergency also remains in effect across the state as hundreds of reinforcements arrive from interstate to bolster firefighters already on the ground.

Meanwhile, police have charged two boys, aged 11 and 15, with deliberately lighting a fire that tore through the Heatherbrae area near Newcastle last week.